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Discussion Starter #1
Hello, I’ve resorted to this forum in hopes I can find what is wrong with my truck. I have a persistent tap or knock coming from what sounds like the front of my engine, I’ve described the issue to my diesel mechanic who told me it was a bad crank pulley, he is the only person who has worked on this truck, is a very trusted man in my community and has experience with the 6.5 as he daily’s one. Hopefully I can attach the video and hopefully you guys can help. The sound responds to engine RPM and continues for a brief second after the engine turns off. It also was very slight after the truck had been sitting for a while but then became louder. Thanks!
 

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It very well could be the Crank pulley and Harmonic Balancer. Both contain rubber within the part and if neglected can eventually break your crankshaft.
Inspect both the Pulley and Harmonic balancer carefully and up close to inspect that rubber.
If one is bad replace them both as a matched set..

Remove your Serp belt. Start and run the engine briefly to see if the noise remains.
If it remains then focus on the HB and crankshaft pulley.
If the noise disappears then check the vacuum pump, It can also make noise when it starts to go bad
 
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The crank pully is a common failure as these trucks age. If you look under the truck at the pully you can see if the rubber inside of it is falling apart, cracking or chunks missing.. Taking off the serpentine belt, You will be able to move the pully back and forth if it is bad. You will also be able to hear the knock as you turn it back and forth.
You should also replace the harmonic balancer, when replacing the pully. Because of the hammering from the bad crank pulley, the balancer is most likely going bad also. If the harmonic balancer goes out. A broken crankshaft may result. It is best to replace both parts at the same time. The knock you hear when you shut off the engine is a tell tale sign of a bad pulley or balancer
 
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Discussion Starter #4
Thanks for the quick responses, I have just buttoned up the crank pulley repair and the one I took out looked fine and the rubber felt new, I glanced at the harmonic balancer and watched it while the engine was spinning, looked fine. I’m going to take it to the mechanic tomorrow and have him listen to it, fortunately he will point me in the right direction instead of charging me for looking at it.
 

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Also, while the belt and fan were off I looked at all of the pulleys and spun them, couldn’t hear anything wrong with any of them.
 

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Run the engine without the fan belt, see if the sound changes.
That is not the crank pulley or HB.
 
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Update, I ran the engine for a quick second without the belt and didn’t hear the sound at all, I read other reports of a few vacuum pumps going bad and sounding like an internal knock. I’m gonna inspect the vacuum pump and try to fill it with oil
 

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Update, I ran the engine for a quick second without the belt and didn’t hear the sound at all, I read other reports of a few vacuum pumps going bad and sounding like an internal knock. I’m gonna inspect the vacuum pump and try to fill it with oil
Interesting I replaced my vacuum pump because of a similar condition. ..
 

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Update, I ran the engine for a quick second without the belt and didn’t hear the sound at all, I read other reports of a few vacuum pumps going bad and sounding like an internal knock. I’m gonna inspect the vacuum pump and try to fill it with oil
you could have run it longer but if the knock was there a few seconds after starting with the belt on then possibly the vacuum pump.
 
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